Various portable electronic apparatuses (cell phones, personal digital assistants PDA, note-type personal computers, DVD players, CD players, digital cameras, digital video cameras, etc.) comprise secondary batteries such as lithium ion batteries as power supplies. Because lithium ion batteries usually generate voltage of about 3.6 V (3.0 V to 4.2 V), electronic apparatuses comprise step-down or step-up DC-DC converters for obtaining operation voltage. The DC-DC converters are used in charging circuits, too.
A DC-DC converter generally has a structure comprising an integrated semiconductor circuit device (active device) comprising switching devices and a control circuit, and passive devices such as inductors, capacitors, etc. mounted on a printed circuit board. However, with the miniaturization of electronic apparatuses requiring the miniaturization of DC-DC converters, proposals have been made to provide small DC-DC converters comprising integrated semiconductor circuit devices mounted on multi-layer substrates containing capacitors and inductor coils.
JP 2005-183890 A discloses, as shown in FIG. 14, a DC-DC converter comprising an integrated semiconductor circuit 106, a capacitor 107, etc. mounted on a laminate 150 containing an inductor coil. The laminate 150 has an upper surface provided with pluralities of external electrodes 111a for mounting the integrated semiconductor circuit 106, etc., and a lower surface provided with pluralities of external electrodes 11 lb for connection to a mounting board. In first wiring layers 101 in the laminate 150, first conductor patterns 109a connected to the external electrodes 111a are formed on pluralities of non-magnetic layers. In coil layers 103 beneath the first wiring layers 101, coil-forming conductor patterns 108 are formed on pluralities of magnetic layers. In second wiring layers 105 beneath the coil layers 103, second conductor patterns 109b connected to the second external electrodes 111b are formed on pluralities of non-magnetic layers. In a peripheral portion of the laminate 150, via-holes 110 penetrating the first wiring layers 101, the coil layers 103 and the second wiring layers 105 are formed.
However, because the integrated semiconductor circuit device 106 is connected to lower surface pads through via-holes 110 in the laminate 150, the via-holes 110 generate parasitic inductance, reducing the efficiency of the DC-DC converter and causing the oscillation of output voltage, resulting in the unstabilized operation. Also, because the integrated semiconductor circuit device 106 is packaged, it is more expensive than a bare-chip type. Further, this DC-DC converter is poor in the heat dissipation of the integrated semiconductor circuit device 106.
JP 2-187054 A discloses, as shown in FIG. 15, a semiconductor apparatus 200 comprising an integrated semiconductor circuit device 201 mounted on a laminate 209 containing an inductor, etc., and sealed by a molding resin 204 together with lead terminals 203 connected by bonding. The semiconductor apparatus 200 has a DIP (dual in-line package)-type package structure, and the lead terminals 203 are connected to terminals 207 on the side surfaces of the laminate 209, and connected to the integrated semiconductor circuit device 201 via bonding wires 202. With this structure, heat in the integrated semiconductor circuit device 201 is conducted to the laminate 209. However, because it is entirely sealed by the molding resin, heat does not dissipate easily. Also, because of smaller lead terminals 203 by the miniaturization of the semiconductor apparatus 200, the lead terminals 203 have an insufficient heat dissipation function. Further, the entirely molded structure is not suitable for miniaturization.
JP 2007-81146 A discloses, as shown in FIG. 16, a QFN (quad flat non-leaded package)-type DC-DC converter 300 comprising a laminate inductor 303 arranged on a conductive support plate 304 continuous to lead terminals 305 and exposed on the outer surface, and a bare-chip-type, integrated semiconductor circuit device 301 arranged thereon, and sealed by a molding resin 309. The lead terminals 305 are connected to electrode pads 302 formed on an upper surface of the integrated semiconductor circuit device 301 via bonding wires 308. The structure having the laminate inductor 303 arranged on the conductive support plate 304 exposed on the outer surface is better than that of JP 2-187054 A in heat dissipation and miniaturization, but such structure makes the DC-DC converter 300 thicker by the thickness of the conductive support plate 304. A thin conductive support plate 304 for making the DC-DC converter 300 thinner has insufficient strength, and a thin inductor 303 has a low Q value and increased magnetic flux leakage.
There is at present a requirement for making semiconductor apparatuses such as DC-DC converters, etc. as thin as 1.0 mm or less in accordance with the thickness of other parts, but the conventional structures cannot be thin without the deterioration of characteristics or cost increase.